1 The Daya Bay Reactor Electron Anti-neutrino Oscillation Experiment Jianglai Liu (for the Daya Bay Collaboration) California Institute of Technology APS DNP Conference, Newport News, Oct. 13, 2007

2 Physics Motivation Weak eigenstate  mass eigenstate  Pontecorvo-Maki-Nakagawa-Sakata Matrix Parametrize the PMNS matrix as: Solar, reactor reactor and accelerator 0  Atmospheric, accelerator  23 ~ 45°  12 = ~ 32°  13 = ?  13 is the gateway of CP violation in lepton sector!

3 Measuring  13 Using Reactor Anti-neutrinos Electron anti-neutrino survival probability e disappearance at short baseline(~2 km): unambiguous measurement of  13 Large oscillation >50 km; negligible <2 km

4 Increase statistics: Use powerful reactors & large target mass & optimized baseline Suppress background: Go deeper underground High performance veto detector to MEASURE the background Reduce systematic uncertainties: Reactor-related: Utilize near and far detectors to minimize reactor-related errors Detector-related: Use “ Identical ” pairs of detectors to do relative measurement Comprehensive program in calibration/monitoring of detectors Previous best experimental limits from Chooz: sin 2 (2  13 ) <0.17 (  m 2 31 =2.5  eV, 90% c.f.) Daya Bay: Goal and Approach Daya Bay: determine sin 2 2  13 with a sensitivity of 1%

5 4 x 20 tons target mass at far site Daya Bay: Powerful reactor by mountains Far site 1615 m from Ling Ao 1985 m from Daya Overburden: 350 m 900 m Daya Bay Near site 363 m from Daya Bay Overburden: 98 m Ling Ao Near site ~500 m from Ling Ao Overburden: 112 m 465 m 295 m 810 m 2  2.9 GW (under construction) 2  2.9 GW in 2010

6 Detection of  e Inverse  -decay in Gd-doped liquid scintillator: 0.3b  + Gd  Gd*  Gd +  ’s (8 MeV) ( t ~30μs)  + p  D +  (2.2 MeV) ( t ~180μs) 50,000b Time, space and energy-tagged signal  suppress background events. Delayed Energy Signal Prompt Energy Signal E   T e MeV 1 MeV8 MeV 6 MeV10 MeV

7 Antineutrino Detector Cylindrical 3-Zone Structure separated by acrylic vessels: I. Target: 0.1% Gd-loaded liquid scintillator, radius=half height= 1.55 m, 20 ton II.  -catcher: liquid scintillator, 42.5 cm thick III. Buffer shielding: mineral oil, 48.8 cm thick 11.6 % 12.5 cm for 8 MeV e - With 192 PMT’s on circumference and reflective reflectors on top and bottom:

8 Calibrating Energy Cuts Automated deployed radioactive sources to calibrate the detector energy and position response within the entire range. 68 Ge (0 KE e + = 2  MeV  ’s) 252 Cf (~4 neutrons/fission, 2.2 MeV n-p and 8 MeV n-Gd captures) LEDs (timing and PMT gains) R=1.35mR=0R=1.775 m In addition, cosmogenic background (n and beta emitters) provide additional energy scale calibration sampled over the entire fiducial volume

9 Muon Veto System Surround detectors with at least 2.5m of water, which shields the external radioactivity and cosmogenic background Water shield is divided into two optically separated regions (with reflective divider, 8” PMTs mounted at the zone boundaries), which serves as two active and independent muon tagger Augmented with a top muon tracker: RPCs Combined efficiency of tracker > 99.5% with error measured to better than 0.25%

10 Backgrounds DYB siteLA siteFar site Fast n / signal 0.1% 9 Li- 8 He / signal 0.3%0.2% Accidental/signal <0.2% <0.1% Background = “prompt”+”delayed” signals that fake inverse-beta events Three main contributors, all can be measured: Background typeExperimental Handle Muon-induced fast neutrons (prompt recoil, delayed capture) from water or rock >99.5% parent “water” muons tagged ~1/3 parent “rock” muons tagged 9 Li/ 8 He (T 1/2 = 178 msec, b decay w/neutron emission, delayed capture) Tag parent “showing” muons Accidental prompt and delay coincidencesSingle rates accurately measured B/S:

11 Baseline Systematics Budget Detector Related0.38% Reactor related0.13% Backgrounds0.3% (Daya Bay near), 0.2% (Ling Ao near and far) Signal statistics0.2% (3 years of running)

12 Daya Bay Sensitivity 90% confidence level, “baseline” detector uncertainties Use rate and spectral shape Milestones DoE CD1 review, April 2007, approved Oct. 13 (Today!) Tunnel construction groundbreaking DoE CD2 review, Jan 2008 July 09 Deployment of the first pair of detectors Sept Begin data taking with near-far

13 Backup

14 Detector-related Baseline: currently achievable relative uncertainty without R&D Goal: expected relative uncertainty after R&D Swapping: can reduce relative uncertainty further